Crystal: homogeneous solid consisting of an element, a chemical compound or an isomorphic mixture, in which the arrangement of the atoms or constituent molecules is regularly repeated according to a precise pattern that allows the creation of a structure called “crystal lattice” and in which we highlight a particular and well-defined external shape delimited by flat faces. The characteristic external shape is due to the regular internal arrangement of the atoms.
Definition from “Dizionario Gemmologico” by Margherita Superchi.
